Electronic devices for producing a given DC voltage from a commercial AC power supply are called a stabilized power supply, and roughly classified into a dropper power supply and a switching power supply. The switching power supply is widely used in general as it is inexpensive and highly efficient.
One conventional switching power supply is denoted at 102 in FIG. 9. The switching power supply 102 comprises a flyback RCC(ringing choke converter)-type switching power supply, and has a diode bridge 120, a smoothing capacitor 121 on a primary side, a transformer 130, and a main switching means 106.
The transformer 130 comprises a primary winding 131, a secondary winding 132, and an auxiliary winding 133 which are arranged so as to be magnetically coupled.
The main switching means 106 comprises an NPN transistor having a collector terminal connected to an end of the primary winding 131. The diode bridge 120 and the smoothing capacitor 121 constitute a rectifying and smoothing means on the primary side for rectifying and smoothing an AC voltage having a commercial frequency and applying the rectified and smoothed voltage between the other end of the primary winding 131 and an emitter terminal of the main switching means 106. Therefore, when a current is supplied to a base terminal of the main switching means 106, rendering it conductive, the primary winding 131 is supplied with a current from the rectifying and smoothing means on the primary side.
The auxiliary winding 133 is connected between the base and emitter terminals of the main switching means 106. When a base current is supplied to render the main switching means 106 conductive and a current starts to flow through the primary winding 131, a voltage is induced across the auxiliary winding 133 for increasing the base current. Therefore, the current flowing through the primary winding 131 increases progressively from the time when the main switching means 106 starts being conductive.
The secondary winding 132 is connected to a rectifying and smoothing means on the secondary side which comprises a diode 135 and a smoothing capacitor. Because of the rectifying action of the diode 135, when the main switching means 106 is rendered conductive and a current flows through the primary winding 131, no current flows through the secondary winding 132, and when the main switching means 106 is cut off and the current flowing through the primary winding 131 stops, a current flows through the secondary winding 132.
FIG. 10 shows operating waveforms of the RCC-type switching power supply 102. The waveform indicated by the reference numeral 151 represents the current flowing through the primary winding 131, and the waveform indicated by the reference numeral 152 represents the current flowing through the secondary winding 132.
The waveform indicated by the reference numeral 153 represents the collector voltage of the main switching means 106, with a low level showing a conducted state thereof and a high level showing a cut-off state thereof. When the main switching means 106 changes from the conducted state to the cut-off state, the energy stored in the primary winding 131 is transferred to the secondary winding 132, causing a current to flow through the secondary winding 132.
The transfer of the energy from the primary winding 131 to the secondary winding 132 is carried out by a magnetic coupling between the primary winding 131 and the secondary winding 132. Since the magnetic coupling is actually not 100%, not all the energy of the primary winding 131 is transferred to the secondary winding 132. With the RCC-type switching power supply, in particular, a large gap is present in the core of the transformer 130, thus lowering the magnetic coupling between the primary winding 131 and the secondary winding 132 thereby increase the leakage inductance of the primary winding 131.
A T-type equivalent circuit of the transformer 130 taking the leakage inductance into account is shown in FIG. 11(a). The reference character L.sub.0 represents an exciting inductance (an inductance which contributes to the magnetic coupling between the primary winding 131 and the secondary winding 132), and the reference characters L.sub.1, L.sub.2 represent leakage inductances, respectively, of the primary winding 131 and the secondary winding 132. In the transformer 130 as a whole, the leakage inductances L.sub.1, L.sub.2 are added to the exciting inductance L.sub.0 in the form of a T.
As can be seen from the equivalent circuit, if the transformer 130 is seen from the primary side thereof, the leakage inductance L.sub.1 and the exciting inductance L.sub.0 are in series with each other. Therefore, when the main switching means 106 is rendered conductive and a current i.sub.1 flows through the primary winding 131, since the current i.sub.1 flows through the leakage inductance L.sub.1 and the exciting inductance L.sub.0, energy is stored in these inductances.
When the main switching means 106 changes from the conducted state to the cut-off state under this condition, the energy stored in the exciting inductance L.sub.0 is transferred to the secondary winding 132, causing a current i.sub.2 to flow through the secondary winding 132. On the other hand, the energy stored in the leakage inductance L.sub.1 is not transferred to the secondary winding 132, but generates an electromotive force across the leakage inductance L.sub.1. Since the electromotive force is actually generated across the primary winding 131 in a direction to maintain the current i.sub.1, it applies a surge voltage as indicated by the reference numeral 155 in FIG. 10 to the collector terminal of the main switching means 106.
The RCC-type switching power supply 102 has a snubber circuit 105 connected to the collector terminal of the main switching means 106. As shown in FIG. 11(b), when a voltage is generated across the primary winding 131 due to the electromotive force across the leakage inductance L.sub.1, a diode 123 is rendered conductive, causing a current i.sub.3 to flow therethrough to charge a snubber capacitor 124. With the snubber capacitor 124 being charged, the energy stored in the leakage inductance L.sub.1 is transferred to the snubber capacitor 124, which absorbs the surge voltage to prevent the main switching means 106 from being destroyed.
When the charging of the snubber capacitor 124 is finished, the snubber capacitor 124 starts being discharged, causing a discharged current i.sub.4 to flow in a closed current path made up of the snubber capacitor 124 and a resistor 125, as shown in FIG. 11(c), for thereby dissipating the energy stored in the leakage inductance L.sub.1 as heat. When the snubber capacitor 124 is charged, since no current is limited, a pulse current indicated by the reference numeral 154 flows through the diode 123.
In the conventional snubber circuit 105, as described above, the snubber capacitor 124 is charged by the energy stored in the leakage inductance L.sub.1. Inasmuch as the energy is finally consumed in its entirety by the resistor 123 and dissipated as heat, the energy stored in the leakage inductance L.sub.1 is not effectively utilized. Consequently, the conventional RCC-type switching power supply 102 has been poor in electric power conversion characteristics and low in efficiency.
In the conventional snubber circuit 105, furthermore, a certain period of time (a time to conduct the diode in the forward direction) is required after the main switching means 106 has changed from the conducted state to the cut-off state, generating an electromotive force across the leakage inductance L.sub.1, until the diode 123 is rendered conductive by the electromotive force, causing the current i.sub.3 to flow. Since the discharging of the snubber capacitor 124 is not finished while the main switching means 106 is being cut off, a voltage which is the sum of the voltage produced by the rectifying and smoothing means on the primary side and the voltage across the snubber capacitor 124 is applied to the collector terminal of the main switching means 106 when the diode 123 is rendered conductive.
Because such a high surge voltage is applied to the collector terminal of the main switching means 106, the main switching means 106 needs to comprise a high-dielectric-strength device capable of withstanding such a surge voltage.
The present invention has been devised to solve the above problems of the prior art. It is an object of the present invention to provide an RCC-type switching power supply which produces a reduced amount of heat and is highly efficient.
Another object of the present invention is to provide an RCC-type switching power supply which does not produce a surge voltage.